Raman spectroscopy measures molecular vibrations, which are determined by the structure and chemical bonding as well as the masses of constituent atoms, molecules, ions, etc. Raman spectra provide us with unique chemical and structural identification. Conventional micro-Raman spectroscopy has a spatial resolution of about 0.5 μm governed by the diffraction limit and even worse for IR spectrometers because of the longer wavelengths. The near-field scanning Raman microscopy (NSRM) exploits an optical fiber tip with a small aperture to deliver laser radiation or collect the scattered light [S. Webster et al., Vibrat. Spectrosc. 18 (1998) 51; E. J. Ayars and H. D. Hallen, Appl. Phys. Lett. 76 (2000) 3911; C. Jahncke et al., Appl. Phys. Lett. 67 (1995) 2483]. The main reason for limited use of NSRM stems from the facts that Raman signals are intrinsically weak because very low laser power can be delivered through a fiber tip (typically, 10−7 W). Another serious drawback of a fiber based delivery or collection systems are parasitic Raman signal resulting from the fiber itself.
An alternative approach to the one based on the use of optical fiber tips is to use apertureless metal tip-mediated SERS which improves significantly the Raman intensity [R. M. Stockle et al., Chem. Phys. Lett. 318 (2000) 131; M. S. Anderson, U.S. patent, Pub. No.: U.S. 2002/0105641 A1; S. Kawata and Y. Inouye, Jpn. Patent No. 3190945 (filed 1992/registered 2001; Y. Inouye and S. Kawata, Opt. Lett., vol. 19, 159 (1994)]. However, the enhancement factor is several orders of magnitude less than the enhancement for SERS in conventional SERS-active substrates (colloid aggregates, electrochemically etched metal surfaces, etc.), and it is restricted by a low quality factor of the plasmon resonance for a single particle (metal tip) used in this approach. The enhancement occurs only within a narrow spectral range.